Mating (or conjugation) in Paramecium occurs between complementary cells during special periods of reactivity. The interaction between reactive cells shows a high degree of specificity. When cells of opposite mating type are mixed under the appropriate conditions, they agglutinate into clumps containing from two to twenty animals. The agglutination phase is followed by a separation of the clumped cells into complementary pairs. Following a reciprocal exchange of gametic nuclei, the cells separate and undergo mitotic division. The agglutinating reaction of Paramecium, which triggers the cellular events leading to nuclear exchange and cell division, is reminiscent of another class of agglutinins - the lectins. The project outlined here is directed toward the isolation and characterization of the mating substances (M-agglutinin) of paramecium. Such characterization should lead to an understanding of the molecular basis for the specificity of agglutination in the mating reaction as a model for describing this specific cell-cell interaction.